1966 ammeter with new engine, alternator, and wiring harness

[mf_luder]

I'm trying to get the ammeter working on a 1966 C10. This truck does not have its original engine, alternator, or wiring harness. I am not interested in replacing the ammeter with a volt meter.

I need to know what voltage produces full deflection in the ammeter. It's probably in the range of 50 - 500 mV. Alternatively, I need to know the resistance of the 10 AWG wire that goes from the horn relay to the positive battery terminal with high precision (it's probably a few milliohms). I'm hoping that somebody can either point me to some documentation that I haven't been able to find myself or actually take one of these measurements.

There's a lot of conflicting information on the Internet about how these ammeters work. As best as I can tell, it's basically like this:

In general, an ammeter works by measuring the voltage across a resistor or "shunt" with a resistance that's low (a few milliohms) and accurately known. In a car, this resistor is placed between the positive battery terminal and the rest of the car (except the starter) so that the voltage across it (less than a volt) will be an indication of how much current is flowing to or from the battery and in which direction (read about ohm's law if that doesn't make sense to you). Early ammeters had built-in resistors and the current being measured ran through the gauge, which can be dangerous with large currents. Newer ones run a fused wire to each end of an external resistor, so that the current flowing through the gauge is small. In 1966 Chevrolet was using the latter type, and the "resistor" was actually the length of stranded 10 AWG wire running from the bus on the horn relay to the positive battery terminal. (Copper wire does have some resistance; Not much, but that's what you want for an ammeter shunt.)

My truck has a 350 with an internally regulated 63 amp alternator on the passenger's side, for which reason the wire running between the battery and the horn relay is getting deleted. Even if it was not going to get deleted, I believe that the shunt resistance needs to change to account for the higher amperage alternator.

If I know the voltage that produces full deflection in the gauge, I can calculate the resistance I need to produce that voltage at any amperage I desire. Alternatively, if I know the resistance of the original shunt, I can calculate the voltage that it would have produced at the original alternator's maximum output and then calculate the resistance I need to produce the same voltage at the new alternator's maximum output.

Low tolerance, high amperage resistors can be a bit pricey, otherwise I would simply start with a low resistance one and and replace it with higher resistance ones until I got the result I wanted. Unfortunately it would be cheaper to buy a DC power supply that I could use to test the gauge. That's what I'll end up doing if nobody here can help.

Thanks for reading.